This invention relates to hydraulic brake control apparatus, and more particularly, to a take-in slack adjusting apparatus.
In U.S. Pat. No. 3,707,309, there is described a hydraulic brake system for railway rapid transit service which is known commercially as the PS-68 Brake Equipment, P.S.B.E., marketed by New York Air Brake Company, a Unit of General Signal Corporation. In that system, the brake cylinders on each truck are provided with a translating and valving unit of the type described in U.S. Pat. No. 3,622,207, granted Nov. 23, 1971, which joins these cylinders to a hand brake control unit and to a hydro-pneumatic booster assembly constructed in accordance with the preferred teachings of U.S. Pat. No. 3,513,656, granted May 26, 1970.
In such a braking system, it is desirable to have a small, predetermined amount of clearance between the brake shoe and its wheel. Hence, the system includes a provision for some amount of hydraulic fluid slack in the hydraulic braking circuit. It is critical that this hydraulic slack neither be too great nor too small. If the slack is too small, then the brake shoe may be in constant contact with the wheel resulting in an undesirable stuck brake condition. On the other hand, if there is too much hydraulic slack, then the shoe might not engage the brake upon operation of the hydro-pneumatic booster, thereby resulting in an undesirable brake failure. In U.S. Pat. No. 3,955,370, granted May 11, 1976, there is described a hydraulic slack adjuster which affords double-acting slack compensation so that it corrects for insufficient as well as excess shoe clearance during cycling of the brakes. In addition, that adjuster enables the shoe clearance to be increased intentionally during maintenance operations by cycling of the brakes.
That slack adjuster essentially includes a cylinder disposed between the hydraulic brakes and the hydro-pneumatic booster. The cylinder includes a differential piston which carries a double-acting check valve. During maintenance operations, the slack between the shoe and the wheel manually increased by retracting the brake cylinder with a pry bar. In effect, all one has to do is to apply a force to the brake shoe that results in a pressure high enough to open the check valve in the piston. Once the check valve is open, hydraulic fluid will drain out of the brake circuit thereby increasing shoe clearance.
Under severe winter conditions, it is possible that snow, ice or sleet may accumulate on the railway right-of-way. It is recommended practice that the right-of-way be cleared by appropriate plows. However, such accumulations may occur before a plow has had time to clear the right-of-way in which case the brake rigging may become a plowing or profile clearing device. When such a condition exists, it is possible that the accumulated snow or ice will cause a considerable force to be applied against the brake shoe thereby forcing open the check valve in the slack adjuster device and forcing an excess amount of fluid out of the brake lines. The hydro-pneumatic booster is only capable of applying a predetermined volume of hydraulic fluid for each service application. Hence, if the oil forced out of the brake lines exceeds the amount of applicable fluid, then these brakes will not be applied upon the initial service of the hydro-pneumatic booster. Under such conditions, it is recommended practice that the hydro-pneumatic booster be applied 2 or 3 times before an expected braking application can occur.
Since an operator or brakeman may fail to follow the recommended application, it would be desirable to have a slack adjusting apparatus which retains all of the desirable features of a double-acting slack adjuster as well as having means for limiting the amount of excess slack in a brake line.